Communication systems are known to comprise mobile transmitters and receivers, such as in-car mobile or hand-held portable radios (mobiles), as well as fixed transmitters and fixed receivers, such as base stations or controllers (fixed end). A typical message within such a communication system may begin with a mobile unit converting an audio signal into a digital data stream suitable for transmission over an RF channel to either another mobile unit or the fixed end. Such systems are often used by public safety institutions, such as local or federal law enforcement agencies. The existence of commercially available radio frequency scanners makes it possible for unauthorized parties to monitor the information transmitted within such a communication system. To reduce unauthorized eavesdropping, communication systems encrypt communications such that, without knowledge of the encryption method and a decryptor, the communications are unintelligible.
As is known, digital encryption methods use a reversible algorithm to introduce randomness into a digital data stream. An algorithm that randomizes digital data is called an encryptor; that which reconstructs the original data from the randomized data, a decryptor. An encryptor/decryptor algorithm typically utilizes dynamic parameters, hereafter referred to as keys, to uniquely specify the nature of the randomness introduced to the digital data stream. Thus, only encryptors and decryptors utilizing an identical algorithm and key are capable of reproducing intelligible messages. To further maintain security, the keys used within such a system are periodically changed, often called rekeying. This must be done in an efficient manner to ensure that all elements within the system are capable of communication with one another.
The use of external devices to change encryption key information within individual communication units is known. Such methods require that each communication unit be brought in from the field for rekeying by a technician. Obviously, this could be impractical in a system with thousands of communication units.
Another method of rekeying communication units is over-the-air rekeying (OTAR). OTAR sends messages which contain the required encryption key information over an RF (radio frequency) channel to each unit. In this way, it is not necessary to co-locate each communication unit with an external rekeying device. In addition, it is also known to use a single, group-wide message to rekey all communication units within a group at once, referred to as group OTAR. Group OTAR is effective if all communication units are within the system's coverage area and are powered-up. If there are communication units that are out of range or powered-down at the time of the group OTAR call, they will not be rekeyed. Therefore, a need exists for a method which allows communication units that were excluded from a group OTAR message to be rekeyed, thus ensuring greater concurrence of encryption key information among all communication units within a system.